With respect to injection valves for internal-combustion engines, very high requirements apply to the precision and robustness of the injection quantity under all operating conditions and over the entire service life of an associated motor vehicle. In order to achieve these objectives, control methods for the injection valves have been developed. In some cases, present-day control concepts utilize feedback signals from the piezoelectric actuator for the purpose of identifying individual static points of the nozzle-needle position during the actual injection process. In this connection, the piezoelectric actuator acts as a sensor. However, this information is subject to considerable disturbance-variable influences, because the piezoelectric actuator is used as an actuator and as a sensor at the same time. Moreover, these so-called signal-based approaches do not permit a statement about the dynamic behavior of the nozzle needle—that is to say, it is not possible to characterize paths of motion of the needle stroke. Consequently, the generation of absolute position values is not possible. But, in the case of injection valves that have no mechanical stop-points (for example, limitation of the nozzle-needle stroke by mechanical blocking), for precise actuation of the injection valve it is important to know the absolute position of the nozzle needle. This is decisive for a precise realization of demanded injection quantities.
Therefore, position values of nozzle needles can currently only be captured statically by utilizing piezoelectric effects (for example, coupling of force between nozzle needle and piezoelectric drive when closing the needle). These methods are subject to considerable disturbance-variable influences which can only be suppressed to a limited extent. In this connection, elaborate plausibility-checking methods find application which, however, under certain circumstances cannot filter out all possible characteristic cases and error cases and consequently result in remaining, impermissible residual errors.
Disturbance variables affecting the feedback signal are generated, inter alia, by the drive profile of the final stage, by the idle stroke in the transmission of force between piezoelectric actuator and nozzle needle, by friction effects in the region of the nozzle needle, and also by the actual stroke behavior of the piezoelectric actuator. The stated influences reduce the robustness of the derived controlled variables and consequently also have an effect on the quality of the control performance and ultimately on the quality of the injection quantity.